It is known to use diffusion coating processes to apply a coating onto a workpiece, in order to improve the surface characteristics or properties of the workpiece, such as the wear resistance and the corrosion resistance thereof. The workpiece may typically consist of an Ni, Co, or Fe based alloy, while the coating layer applied by the diffusion coating process typically comprises Al, Cr or Si, for example.
In the prior art, diffusion coating processes are known in which the coating is applied by means of a powder pack in which the workpiece is embedded for carrying out the process. The coating material is to be transported and applied onto the workpiece from the powder pack by a vapor phase transport, and also possibly by some degree of liquid phase transport, within the powder pack. Such processes are typically known as "pack cementation" or "pack diffusion" processes. For example, U.S. Pat. No. 3,667,985 discloses such a process for applying a coating of AlTi, and U.S. Pat. No. 3,958,047 discloses such a process for applying a coating of Cr. These processes are basically suitable for coating workpieces made of heat resistant alloys in a uniform manner with a coating having a high proportional content of Al or Cr.
However, all of these known powder pack processes suffer certain process dependent disadvantages. Namely, for example, the maximum size of the powder pack, and thus the maximum size of the workpiece or workpieces, is limited due to problems of thermal conduction for forming a uniform coating layer. Furthermore, the source or donor powder in the powder pack undergoes sinter-bonding onto the surface of the workpiece, and/or otherwise forms an unacceptable roughness of the surface of the workpiece due to embedding of the powder in the coating layer. The handling of the powder is environmentally problematic due to the emission of dust and the like and the problems involved in breaking down and disposing or recycling the fractional powder mixtures.
Certain processes for carrying out diffusion coating without using a powder pack are also known, for example as disclosed in European Patent Publication 0,480,867 A2 and British Patent Publication 1,135,015. Both of these known processes, however, suffer the disadvantage that the reactions for forming the donor or source gas and for achieving the deposition, are carried out in a thermodynamically inefficient or disadvantageous manner, due to the structural arrangement of the apparatus, and the geometric arrangement of the workpiece and the donor source within the apparatus. A further disadvantage of these known coating processes in comparison to the powder pack processes, is that the coating layer has a smaller thickness and/or a smaller proportional content of the diffused elements. Another disadvantage of these known methods is that the circuit arrangement of the donor gas source before or upstream of the reaction chamber requires the use of various interference-sensitive auxiliary apparatus for achieving a gas mixture that is properly reactable.